1. Field of the Invention
The present invention relates to wiring and an organic field effect transistor (FET), which are formed using a printing technique, and a method of manufacturing the same.
2. Description of the Related Art
A thin display device uses a liquid crystal or an organic electro luminescence (EL) element. In the thin display device, thin film transistors (TFTs) are used as elements driving pixels. The TFTs each use amorphous silicon or polycrystalline silicon as a channel. Meanwhile, in order to improve flexibility and reduce the manufacturing cost, studies have been extensively made on a display device using organic EL elements as pixels for the purpose of forming a TFT (used for a driving circuit) using an organic material. TFTs using amorphous silicon or polycrystalline silicon have poor flexibility. Thus, display devices with such TFTs also have poor flexibility. In addition, a vacuum facility is used in the manufacturing process for the TFTs, which increases the manufacturing cost. However, if a TFT can be formed using an organic material, this makes it possible to achieve a display device with high flexibility. Furthermore, depending on the type of organic material used, the TFT formed using the organic material can be manufactured by a wet process such as a printing technique. This method does not require a vacuum facility for manufacturing the TFT, which reduces the manufacturing cost.
Organic molecules are mainly divided into two categories: organic molecules with low molecular weight such as a monomer and oligomer; and organic molecules with high molecular weight, which are categorized as polymers. TFTs having a channel composed of organic molecules (which is an organic TFT) are also mainly divided into two types depending on which type of molecules is used to form a channel: an organic TFT with a channel composed of a monomer or the like; and an organic TFT with a channel composed of a polymer. In the case of the organic TFT with a channel composed of a monomer or the like, it has been demonstrated that the mobility of carriers flowing in the channel is increased to a similar level to the mobility of carriers in amorphous silicon when the organic molecules maintain good crystalline properties. Thus, it is relatively easy to achieve an organic TFT with a high operation speed. It is difficult, however, to reduce the manufacturing cost since a molecular beam epitaxy method is generally used for organic molecules to form a channel. In contrast, it is easy to apply a wet process to an organic TFT with a channel composed of a polymer, which reduces the manufacturing cost. However, the mobility of carriers flowing in the channel composed of a polymer is approximately one tenth that in the organic TFT with a channel composed of a monomer or the like. Therefore, the organic TFT with a channel composed of a polymer provides low performance.
In general, there is a problem in that the operation speed of a TFT with a channel composed of organic molecules is lower than that of silicon-based TFT. This is because the mobility of carriers flowing in the channel composed of organic molecules is low. It is known that one of the main reasons is the scattering of carriers in the channel. For a monomer or the like, in order to reduce the carrier scattering, the following method is used in many cases: the size of grains in a crystal forming a channel is increased so as to reduce the number of grain boundaries through which carriers flow between electrodes disposed at both ends of the channel. This reduces carrier scattering. The use of a single crystal for a channel is most desirable since the effect of the grain boundaries can be reduced. In the case of a channel composed of a polymer, if a polymer is extended to a maximum extent in a direction parallel to the flow of carriers in a channel, the carrier scattering in the polymer is reduced in general.
To realize a display device with high flexibility, a peripheral circuit driving pixels also requires high flexibility. TFTs used in a circuit driving pixels require carrier mobility of about 10 cm2/Vs. Currently, however, only a TFT with a channel composed of organic molecules with low molecular weight has been demonstrated to meet the above requirement. For example, Non-Patent Document 1 (Science, 303, 1644 (2004)) describes that carrier mobility of 15 cm2/Vs is obtained in an organic TFT with a channel composed of a single crystal of rubrene molecules. Non-Patent Document 2 (Applied Physics Letters, 84, 3061 (2004)) discloses that carrier mobility of 35 cm2/Vs is achieved for a single crystal of highly-purified pentacene molecules at a room temperature. Such high carrier mobility, however, can be obtained for a single crystal sample by paying special attention to the manufacturing of the single crystal or TFT sample. When a monomer or the like is used to form a channel, a thin film crystal of organic molecules, which is formed by the molecular beam epitaxy method, is more generally used. In this case, it is difficult to form a single crystal for the channel. In addition, a vacuum apparatus is required, which causes disadvantages in cost and mass productivity.
Such organic TFTs pose problems that the following two types of requirements cannot be satisfied simultaneously: performance required for application; and manufacturing cost and mass productivity required for production profitability. More specifically, the molecular beam epitaxy method is generally used to manufacture an organic TFT including a monomer or the like, which will easily improve the TFT performance. In this case, there are disadvantages in the production. On the other hand, it is relatively easy to reduce the manufacturing cost for an organic TFT including a polymer. In this case, since the TFT performance is very low, such a TFT has only limited applications.
To solve the above problems, there is a method in which a monomer or the like is dissolved in a solvent and printed so as to form a semiconductor layer used as a channel. For example, Non-Patent Document 3 (Journal of Applied Physics, 79, 2136 (1996)) describes pentacene molecules, which are a most typical example of organic molecules applied to a TFT including a monomer or the like. In addition, Non-Patent Document 4 (Journal of American Chemical Society, 124, 8812 (2002)) describes a technique for synthesizing derivatives of pentacene molecules to form a thin film using a solution with increased solubility for a solvent. Non-Patent Document 5 (Synthetic Metals, 153, 1 (2005)) discloses a technique for dissolving pentacene molecules directly in a solvent and coating them so as to form a thin film. Furthermore, Non-Patent Document 2 and Non-Patent Document 6 (Japanese Journal of Applied Physics, 43, L315 (2004) describe a technique for dissolving pentacene molecules in an organic solvent. Those techniques make it possible to form an organic film composed of a monomer or the like by printing without using a vacuum apparatus, which increases the possibility of achieving a TFT formed with required performance and low cost.
In order to form an organic TFT by printing with a low cost, it is desirable that an organic semiconductor, wiring and electrodes formed with metal lines be formed by printing. To form the above, the metal is made into particles. Then, the particles are covered with an organic material and the like so as to have solubility to a solvent. The dissolved particles are turned into metal ink or paste and diffused to specified locations by printing. Then, the organic material is removed by a treatment at a required temperature to form the metal wiring and electrodes. The method of forming wiring with silver or gold paste by printing has been established.